Investments Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 1,389,662	$ 1,234,182
Total Loans	1,389,662	1,234,182
Non-Accrual Status	0	0
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial Mortgage Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,330,465	1,149,947
Total Loans	1,330,465	1,149,947
Non-Accrual Status	0	0
Commercial Mortgage Loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial Mortgage Loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial Mortgage Loans | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Agricultural Property Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	59,197	84,235
Total Loans	59,197	84,235
Non-Accrual Status	0	0
Agricultural Property Loans | 30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Agricultural Property Loans | 60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Agricultural Property Loans | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 0	$ 0